Two million people are suffering from hepatitis C virus (HCV) in Japan, among them 36,000 people develop hepatocellular carcinoma every year where many of said cancer patients result in death. Although interferon (IFN) is currently used as an anti-HCV drug, its effect is limited and associated with serious side effects. Therefore, there has been a need for the development of a safer and more effective drug. Moreover, urgent action is needed since the risk of developing cancer is increasing with the aging of the infected patients.
Under the current circumstances, drugs for suppressing replication of HCV virus such as nucleic acid analogs and protease inhibitors have been developed and used for the treatment. When these drugs are used for the treatment, however, a drug-resistant virus is likely to emerge and the action mechanism of the drugs suggests that complete elimination of the virus is difficult and thus lifelong medication is required. Recently, a highly active anti-HCV drug has been developed and used but the treatment cost thereof is very high as 800,000-1,200,000 yen per cycle, limiting the number of patients who can receive this treatment. Under such a circumstance, establishment of a safe and inexpensive curative treatment that allows withdrawal or release from lifelong medication has been strongly required.
The present inventors have established research resources relating to HCV research and established various model experimental systems by preparing infectious HCV cDNA clones and establishing infected animals such as transgenic mice and human liver chimeric mice that can develop hepatitis C (for example, see Non-patent document 1: Wakita T., et al., J. Biol. Chem., 1998, vol. 273, pp. 9001-9006). Major features of HCV infection include a high rate of persistent infection and transfer to development of chronic hepatitis. For a long period of time, the present inventors have devoted themselves to earnest analysis and research on this mechanism in terms of acquirement and deficit in immune tolerance using the above-described model experimental system and else (for example, see Non-patent document 2: Inoue K., et al., Hepatology, 2007, vol. 45, pp. 921-928).
Many attempts have been made so far to develop a vaccine for preventing HCV infection but there has been no achievement in completely preventing the infection (for example, see Non-patent document 3: Choo Q L., et al., Pros. Natl. Acad. Sci. 1994, vol. 91, pp. 1294-1298, Non-patent document 4: Puig M., et. al., Vaccine 2004, vol. 22, pp. 991-1000, Non-patent document 5: Abraham J D., Vaccine 2004, vol. 22, pp. 3917-3928 and Non-patent document 6: Elmowalid G A., et. al., Pros. Natl. Acad. Sci. 2007, vol. 104, pp. 8427-8432).
In general, a so-called prime-boost method is known in as a method for strongly inducing cell-mediated immunity, in which a primary immunization is performed with a DNA vaccine and then a boost immunization with a recombinant vaccinia virus (vaccinia vaccine) (for example, see Non-patent document 7: Houghton M., Immunol. Rev., 2011 January, vol. 239(1), p. 99-108).